The present invention relates to a surface mounting structure for a surface mounting electronic component.
Recently, a surface mounting technique has been used wherein the electrodes of an electronic component are connected to wiring patterns on the surface of a substrate as part of the process for manufacturing a high density of electronic devices on the substrate. FIGS. 4A-4C are top views showing conventional surface mounting structures. These structures are in the form of chip components 42 such as chip resistors, capacitors and the like which are connected through solder to wide patterns 41 as an electrical connection pattern formed on a metal substrate 40.
A case is assumed where the surface mounting structures in FIGS. 4A-4C are incorporated into an in-vehicle electronic circuit which is assumed to be used under severe temperature conditions such as from sub-zero degree to several tens of degrees Celsius. When each of the wide patterns 41, the chip components 42 and the like repeatedly expand and contract due to temperature changes, a crack may be initiated in a solder 43 between the upper surface of the wide pattern 41 and the bottom surface of the chip component 42, as shown, for example, in FIG. 5A. The crack may eventually propagate through the solder 43, leading to the possibility that the wide pattern 41 and the chip component 42 become electrically disconnected from each other, thereby resulting in an electrically open state.
Conventionally, to prevent the formation of a crack in the solder 43, the volume of the solder 43 supplied is adjusted so that the solder 43 is formed into a fillet matching the size of the chip component 42 as shown by FIG. 5B. Furthermore, the shape of the land to which the bottom surface of the chip component 42 is attached with solder is modified to minimize stress of the solder between the bottom surface of the chip component 42 and the land where a crack may be generated initially (cf. Unexamined Japanese Patent Publication No. 2005-228885). Despite improvements shown in FIG. 5B, there remains the possibility that a crack may be generated and subsequently propagate through the solder 43.
Thus the present invention is directed to an improved surface mounting structure for a surface mounting electronic component which prevents crack formation due to thermal stress in the solder connecting the electronic component 42 and the wide pattern 41.